Aluminum-induced distortion in calcium signaling involving oxidative bursts and channel regulation in tobacco BY-2 cells.

Trivalent cations such as those of Al, La, and Gd are phytotoxic. Our previous works showed that addition of LaCl(3) or GdCl(3) to tobacco cells triggers the generation of superoxide (O(2)-). Here, we show that AlCl(3) at normal physiological pH (5.8) induces much greater production of O(2)- (detected with a specific chemiluminescence probe), indicating that these trivalent cations similarly induce the oxidative bursts. It was shown that NADPH oxidase is involved in the generation of O(2)- and the yield of O(2)- was dose-dependent (ca. 6mM Al, optimal). Following the acute spike of O(2)-, a gradual increase in cytosolic-free Ca(2+) concentration (Ca(2+)) was detected with the luminescence of recombinant aequorin over-expressed in the cytosol. Interestingly, a O(2)- scavenger and a Ca(2+) chelator significantly lowered the level of Ca(2+) increase, indicating that the Al-induced O(2)- stimulates the influx of Ca(2+). Compared to the induction of O(2)- generation, the Ca(2+) elevation was shown to be maximal (340 nM) at relatively lower Al concentrations (ca. 1.25 mM). Thus, the Al concentration optimal for O(2)- is too much (inhibitory) for Ca(2+). In addition, high concentrations of Al were shown to be inhibitory to the H(2)O(2)-induced Ca(2+) influx. This explains the ineffectiveness of high Al concentration in the oxidative burst-mediated induction of Ca(2+) increase. It is likely that Al-induced Ca(2+) elevation is manifested from the finely geared balance between the O(2)- -mediated driving force and the channel inhibition-mediated brake. Furthermore, it is note-worthy that Al (< or =10mM) showed no inhibitory effect on the hypo-osmolarity-induced Ca(2+) influx, implying that Al may be a selective inhibitor of redox-responsive Ca(2+) channels. Possible target channels of Al actions are discussed.